Rate-distortion optimization of the image compression algorithm based on the warped discrete cosine transform

Recently, an image compression algorithm based on the warped discrete cosine transform has been proposed (Cho and Mitra, IEEE Trans Circuits Systems Video Technol 10 (8) (2000) 1364). The algorithm outperforms JPEG, especially for images with high frequency contents and compound images consisting of picture and text. In this paper, we improve the compression performance by optimizing the parameters of WDCT in the rate–distortion (R–D) sense, whereas the algorithm of Cho and Mitra considers distortion only. The optimization algorithm 9nds the WDCT matrices for each block which minimize the distortion for the given bit budget. The problem is formulated in a form that can be solved by the Lagrange multiplier technique. The major di;erence of the proposed algorithm from the conventional optimized JPEG is that the proposed algorithm controls the rate and distortion by warping the frequency contents of input signals before DCT, whereas the conventional optimized JPEG controls the R–D by manipulating the data after DCT, i.e. by adjusting the quantization matrix. The simulation results show that the proposed algorithm yields better performance than the conventional optimized JPEG at high bit rates and for images with high frequency components. Moreover, the proposed algorithm requires lower computational complexity, because only a single parameter is controlled to meet the R–D constraints. ? 2003 Elsevier B.V. All rights reserved.